The present invention relates to a reactivity control arrangement and a fast reactor utilizing the same.
Japanese Patent No. 3126524 (Patent Document 1) discloses a known fast reactor (entire contents of which are incorporated herein by reference). Referring to FIG. 10 of the accompanying drawings, the known fast reactor comprises a reactor core 2 that has nuclear fuel assemblies. The reactor core 2 shows a substantially cylindrical profile and a core barrel 3 is arranged around the outer periphery of the reactor core 2 in order to protect the reactor core 2. A circular-shaped annular reflector 4 is so provided around the core barrel 3 as to surround the core barrel 3. A bulkhead 6 is arranged at the outside of the reflector 4 to surround the reflector 4 and constitutes the inner wall of a coolant flow channel for primary coolant. A reactor vessel 7 that functions as the outer wall of the coolant flow channel is arranged at the outside of the bulkhead 6 with a gap interposed between them. A neutron shield 8 is arranged in the coolant flow channel to surround the reactor core 2.
A hexagonal barrel-shaped wrapper tube is arranged at the center of the reactor core and a cylindrical reactor shutdown rod and six sectorial neutron absorbers are contained in the wrapper tube, the neutron absorbers surrounding the shutdown rod. The reactor shutdown rod operates as backup reactor shutdown system for the reflector that functions as main reactor shutdown member, whereas the neutron absorbers suppress the reactivity when the initial surplus reactivity is high. Of the drive system, the reactor shutdown rod drive mechanism comprises a drive section and an upper guide tube and links to the reactor shutdown rod arranged below the upper guide tube. It has an up and down drive function in normal operation, a scram function in emergency and a reactor shutdown rod releasing/grasping function before the re-start-up. The neutron absorber drive mechanism includes an operation shaft and a hold mechanism, and is adapted to grasp the neutral absorbers only once during the service life of the plant and pull up and hold them individually.
A fast reactor controlled by means of the reflector requires a reactor shutdown rod drive mechanism of the backup reactor shutdown system and a neutron absorber drive mechanism for suppressing an initial surplus reactivity. However, a reflector drive mechanism, an intra-reactor liquid level meter, an intra-reactor thermometer and other instruments are mounted in an upper part of a nuclear reactor, and hence such drive mechanisms need to be installed in a limited space.
Additionally, a fast reactor controlled by means of a reflector can be driven to operate continuously for a long period of time so that the servicing operation needs to be minimized for standing activation devices and the fast reactor requires backup activation mechanisms for unexpected troubles. Particularly, it is desirable that the reactor shutdown rod drive mechanism is equipped with a back up scram mechanism from the viewpoint of safety.
On the other hand, the neutron absorber drive mechanism needs to be arranged adjacent to the reactor shutdown rod so that it is indispensably required to be structurally compact.